NAME

SYNOPSIS

OVERVIEW

This manual describes flex, a tool for generating
programs that perform pattern-matching on text. The manual
includes both tutorial and reference sections:

Description
a brief overview of the tool
Some Simple Examples
Format Of The Input File
Patterns
the extended regular expressions used by flex
How The Input Is Matched
the rules for determining what has been matched
Actions
how to specify what to do when a pattern is matched
The Generated Scanner
details regarding the scanner that flex produces;
how to control the input source
Start Conditions
introducing context into your scanners, and
managing

DESCRIPTION

flex is a tool for generating scanners:
programs which recognized lexical patterns in text.
flex reads the given input files, or its standard
input if no file names are given, for a description of a
scanner to generate. The description is in the form of pairs
of regular expressions and C code, called rules. flex
generates as output a C source file, lex.yy.c, which
defines a routine yylex(). This file is compiled and
linked with the -lfl library to produce an
executable. When the executable is run, it analyzes its
input for occurrences of the regular expressions. Whenever
it finds one, it executes the corresponding C
code.

SOME SIMPLE EXAMPLES

First some simple examples to get the flavor of how one uses
flex. The following flex input specifies a
scanner which whenever it encounters the string
''

%%
username printf(
By default, any text not matched by a flex scanner is copied to the output, so the net effect of this scanner is to copy its input file to its output with each occurrence of pattern and the action. The ''

Here's another simple example:

int num_lines = 0, num_chars = 0;
%%
n ++num_lines; ++num_chars;
. ++num_chars;
%%
main()
{
yylex();
printf(
This scanner counts the number of characters and the number of lines in its input (it produces no output other than the final report on the counts). The first line declares two globals, yylex() and in the main() routine declared after the second

A somewhat more complicated example:

/* scanner for a toy Pascal-like language /
%{
/ need this for the call to atof() below */

include

This is the beginnings of a simple scanner for a language like Pascal. It identifies different types of tokens and reports on what it has seen.

The details of this example will be explained in the
following sections.

FORMAT OF THE INPUT FILE

The flex input file consists of three sections,
separated by a line with just %% in it:

definitions
%%
rules
%%
user code
The definitions section contains declarations of simple name definitions to simplify the scanner specification, and declarations of start conditions, which are explained in a later section.

([0-9?)+
and matches one-or-more digits followed by a '.' followed by zero-or-more digits.

The rules section of the flex input contains a
series of rules of the form:

pattern action
where the pattern must be unindented and the action must begin on the same line.

See below for a further description of patterns and
actions.

Finally, the user code section is simply copied to
lex.yy.c verbatim. It is used for companion routines
which call or are called by the scanner. The presence of
this section is optional; if it is missing, the second
%% in the input file may be skipped,
too.

In the definitions and rules sections, any indented
text or text enclosed in %{ and %} is copied
verbatim to the output (with the %{}'s removed). The %{}'s
must appear unindented on lines by themselves.

In the rules section, any indented or %{} text appearing
before the first rule may be used to declare variables which
are local to the scanning routine and (after the
declarations) code which is to be executed whenever the
scanning routine is entered. Other indented or %{} text in
the rule section is still copied to the output, but its
meaning is not well-defined and it may well cause
compile-time errors (this feature is present for
POSIX compliance; see below for other such
features).

In the definitions section (but not in the rules section),
an unindented comment (i.e., a line beginning with

PATTERNS

The patterns in the input are written using an extended set
of regular expressions. These are:

x match the character 'x'
. any character (byte) except newline
[xyz? a
Note that inside of a character class, all regular expression operators lose their special meaning except escape ('') and the character class operators, '-', ']', and, at the beginning of the class, '^'.

The regular expressions listed above are grouped according
to precedence, from highest precedence at the top to lowest
at the bottom. Those grouped together have equal precedence.
For example,

foo|bar*
is the same as

(foo)|(ba(r*))
since the '*' operator has higher precedence than concatenation, and concatenation higher than alternation ('|'). This pattern therefore matches either the string or the string ''

foo|(bar)*
and to match zero-or-more

(foo|bar)*
In addition to characters and ranges of characters, character classes can also contain character class expressions. These are expressions enclosed inside __ and __:? delimiters (which themselves must appear between the '[' and '?' of the character class; other elements may occur inside the character class, too). The valid expressions are:

alnum:?alpha:?blank:?cntrl:?digit:?graph:?lower:?print:?punct:?space:?upper:?xdigit:?
These expressions all designate a set of characters equivalent to the corresponding standard C isXXX function. For example, alnum:? designates those characters for which isalnum() returns true - i.e., any alphabetic or numeric. Some systems don't provide isblank(), so flex defines blank:? as a blank or a tab.

For example, the following character classes are all
equivalent:

alnum:?]
alpha:?digit:?]
alpha:?[0-9?]
[a-zA-Z0-9?
If your scanner is case-insensitive (the -i flag), then upper:? and lower:? are equivalent to alpha:?.

Some notes on patterns:

-

A negated character class such as the example
will match a newline unless

-

A rule can have at most one instance of trailing context
(the '/' operator or the '$' operator). The start condition,
'^', and

The following are illegal:

foo/bar$
Note that the first of these, can be written

The following will result in '$' or '^' being treated as a
normal character:

foo|(bar$)
foo|^bar
If what's wanted is a

foo |
bar$ /* action goes here */
A similar trick will work for matching a foo or a bar-at-the-beginning-of-a-line.

HOW THE INPUT IS MATCHED

When the generated scanner is run, it analyzes its input
looking for strings which match any of its patterns. If it
finds more than one match, it takes the one matching the
most text (for trailing context rules, this includes the
length of the trailing part, even though it will then be
returned to the input). If it finds two or more matches of
the same length, the rule listed first in the flex
input file is chosen.

Once the match is determined, the text corresponding to the
match (called the token) is made available in the
global character pointer yytext, and its length in
the global integer yyleng. The action
corresponding to the matched pattern is then executed (a
more detailed description of actions follows), and then the
remaining input is scanned for another match.

If no match is found, then the default rule is
executed: the next character in the input is considered
matched and copied to the standard output. Thus, the
simplest legal flex input is:

%%
which generates a scanner that simply copies its input (one character at a time) to its output.

Note that yytext can be defined in two different
ways: either as a character pointer or as a character
array. You can control which definition flex
uses by including one of the special directives
%pointer or %array in the first (definitions)
section of your flex input. The default is %pointer,
unless you use the -l lex compatibility option, in
which case yytext will be an array. The advantage of
using %pointer is substantially faster scanning and
no buffer overflow when matching very large tokens (unless
you run out of dynamic memory). The disadvantage is that you
are restricted in how your actions can modify yytext
(see the next section), and calls to the unput()
function destroys the present contents of yytext,
which can be a considerable porting headache when moving
between different lex versions.

The advantage of %array is that you can then modify
yytext to your heart's content, and calls to
unput() do not destroy yytext (see below).
Furthermore, existing lex programs sometimes access
yytext externally using declarations of the

This definition is erroneous when used with %pointer, but correct for %array.

%array defines yytext to be an array of
YYLMAX characters, which defaults to a fairly large
value. You can change the size by simply #define'ing
YYLMAX to a different value in the first section of
your flex input. As mentioned above, with
%pointer yytext grows dynamically to accommodate
large tokens. While this means your %pointer scanner
can accommodate very large tokens (such as matching entire
blocks of comments), bear in mind that each time the scanner
must resize yytext it also must rescan the entire
token from the beginning, so matching such tokens can prove
slow. yytext presently does not dynamically
grow if a call to unput() results in too much text
being pushed back; instead, a run-time error
results.

Also note that you cannot use %array with C++ scanner
classes (the c++ option; see below).

ACTIONS

Each pattern in a rule has a corresponding action, which can
be any arbitrary C statement. The pattern ends at the first
non-escaped whitespace character; the remainder of the line
is its action. If the action is empty, then when the pattern
is matched the input token is simply discarded. For example,
here is the specification for a program which deletes all
occurrences of

%%
(It will copy all other characters in the input to the output since they will be matched by the default rule.)

Here is a program which compresses multiple blanks and tabs
down to a single blank, and throws away whitespace found at
the end of a line:

%%
[ t?+ putchar( ' ' );
[ t?+$ /* ignore this token */
If the action contains a '{', then the action spans till the balancing '}' is found, and the action may cross multiple lines. flex knows about C strings and comments and won't be fooled by braces found within them, but also allows actions to begin with %{ and will consider the action to be all the text up to the next %} (regardless of ordinary braces inside the action).

An action consisting solely of a vertical bar ('|') means

Actions can include arbitrary C code, including
return statements to return a value to whatever
routine called yylex(). Each time yylex() is
called it continues processing tokens from where it last
left off until it either reaches the end of the file or
executes a return.

Actions are free to modify yytext except for
lengthening it (adding characters to its end--these will
overwrite later characters in the input stream). This
however does not apply when using %array (see above);
in that case, yytext may be freely modified in any
way.

Actions are free to modify yyleng except they should
not do so if the action also includes use of yymore()
(see below).

There are a number of special directives which can be
included within an action:

-

ECHO copies yytext to the scanner's
output.

-

BEGIN followed by the name of a start condition
places the scanner in the corresponding start condition (see
below).

-

REJECT directs the scanner to proceed on to the
yytext
and yyleng set up appropriately. It may either be one
which matched as much text as the originally chosen rule but
came later in the flex input file, or one which
matched less text. For example, the following will both
count the words in the input and call the routine special()
whenever ''

int word_count = 0;
%%
frob special(); REJECT;
[^ tn?+ ++word_count;
Without the REJECT, any REJECT's are allowed, each one finding the next best choice to the currently active rule. For example, when the following scanner scans the token __

%%
a |
ab |
abc |
abcd ECHO; REJECT;
.|n /* eat up any unmatched character */
(The first three rules share the fourth's action since they use the special '|' action.) REJECT is a particularly expensive feature in terms of scanner performance; if it is used in any of the scanner's actions it will slow down all of the scanner's matching. Furthermore, REJECT cannot be used with the -Cf or -CF options (see below).

Note also that unlike the other special actions,
REJECT is a branch; code immediately following
it in the action will not be executed.

-

yymore() tells the scanner that the next time it
matches a rule, the corresponding token should be
appended onto the current value of yytext
rather than replacing it. For example, given the input
__

%%
mega- ECHO; yymore();
kludge ECHO;
First yytext so the ECHO for the

Two notes regarding use of yymore(). First,
yymore() depends on the value of yyleng
correctly reflecting the size of the current token, so you
must not modify yyleng if you are using
yymore(). Second, the presence of yymore() in
the scanner's action entails a minor performance penalty in
the scanner's matching speed.

-

yyless(n) returns all but the first n
characters of the current token back to the input stream,
where they will be rescanned when the scanner looks for the
next match. yytext and yyleng are adjusted
appropriately (e.g., yyleng will now be equal to
n ). For example, on the input
''

%%
foobar ECHO; yyless(3)?;
[a-z?+ ECHO;
An argument of 0 to yyless will cause the entire current input string to be scanned again. Unless you've changed how the scanner will subsequently process its input (using BEGIN, for example), this will result in an endless loop.

Note that yyless is a macro and can only be used in
the flex input file, not from other source
files.

-

unput(c) puts the character c back onto the
input stream. It will be the next character scanned. The
following action will take the current token and cause it to
be rescanned enclosed in parentheses.

{
int i;
/* Copy yytext because unput() trashes yytext */
char *yycopy = strdup( yytext );
unput( ')' );
for ( i = yyleng - 1; i
Note that since each unput() puts the given character back at the beginning of the input stream, pushing back strings must be done back-to-front.

An important potential problem when using unput() is
that if you are using %pointer (the default), a call
to unput()destroys the contents of
yytext, starting with its rightmost character and
devouring one character to the left with each call. If you
need the value of yytext preserved after a call to
unput() (as in the above example), you must either
first copy it elsewhere, or build your scanner using
%array instead (see How The Input Is
Matched).

Finally, note that you cannot put back EOF to attempt
to mark the input stream with an end-of-file.

-

input() reads the next character from the input
stream. For example, the following is one way to eat up C
comments:

%%
(Note that if the scanner is compiled using C++, then input() is instead referred to as yyinput(), in order to avoid a name clash with the C++ stream by the name of input.)

-

YY_FLUSH_BUFFER flushes the scanner's internal buffer
so that the next time the scanner attempts to match a token,
it will first refill the buffer using YY_INPUT (see
The Generated Scanner, below). This action is a special case
of the more general yy_flush_buffer() function,
described below in the section Multiple Input
Buffers.

-

yyterminate() can be used in lieu of a return
statement in an action. It terminates the scanner and
returns a 0 to the scanner's caller, indicating
yyterminate() is also called
when an end-of-file is encountered. It is a macro and may be
redefined.

THE GENERATED SCANNER

The output of flex is the file lex.yy.c, which
contains the scanning routine yylex(), a number of
tables used by it for matching tokens, and a number of
auxiliary routines and macros. By default, yylex() is
declared as follows:

int yylex()
{
... various definitions and the actions in here ...
}
(If your environment supports function prototypes, then it will be

define YY_DECL float lexscan( a, b ) float a, b;

to give the scanning routine the name lexscan, returning a float, and taking two floats as arguments. Note that if you give arguments to the scanning routine using a K''

Whenever yylex() is called, it scans tokens from the
global input file yyin (which defaults to stdin). It
continues until it either reaches an end-of-file (at which
point it returns the value 0) or one of its actions executes
a return statement.

If the scanner reaches an end-of-file, subsequent calls are
undefined unless either yyin is pointed at a new
input file (in which case scanning continues from that
file), or yyrestart() is called. yyrestart()
takes one argument, a FILE * pointer (which can be
nil, if you've set up YY_INPUT to scan from a source
other than yyin), and initializes yyin for
scanning from that file. Essentially there is no difference
between just assigning yyin to a new input file or
using yyrestart() to do so; the latter is available
for compatibility with previous versions of flex, and
because it can be used to switch input files in the middle
of scanning. It can also be used to throw away the current
input buffer, by calling it with an argument of yyin;
but better is to use YY_FLUSH_BUFFER (see above).
Note that yyrestart() does not reset the start
condition to INITIAL (see Start Conditions,
below).

If yylex() stops scanning due to executing a
return statement in one of the actions, the scanner
may then be called again and it will resume scanning where
it left off.

By default (and for purposes of efficiency), the scanner
uses block-reads rather than simple getc() calls to
read characters from yyin. The nature of how it gets
its input can be controlled by defining the YY_INPUT
macro. YY_INPUT's calling sequence is
__max_size characters in the character
array buf and return in the integer variable
result either the number of characters read or the
constant YY_NULL (0 on Unix systems) to indicate EOF. The
default YY_INPUT reads from the global file-pointer

A sample definition of YY_INPUT (in the definitions section
of the input file):

When the scanner receives an end-of-file indication from
YY_INPUT, it then checks the yywrap() function. If
yywrap() returns false (zero), then it is assumed
that the function has gone ahead and set up yyin to
point to another input file, and scanning continues. If it
returns true (non-zero), then the scanner terminates,
returning 0 to its caller. Note that in either case, the
start condition remains unchanged; it does not revert
to INITIAL.

If you do not supply your own version of yywrap(),
then you must either use %option noyywrap (in which
case the scanner behaves as though yywrap() returned
1), or you must link with -lfl to obtain the default
version of the routine, which always returns 1.

Three routines are available for scanning from in-memory
buffers rather than files: yy_scan_string(),
yy_scan_bytes(), and yy_scan_buffer(). See the
discussion of them below in the section Multiple Input
Buffers.

The scanner writes its ECHO output to the
yyout global (default, stdout), which may be
redefined by the user simply by assigning it to some other
FILE pointer.

START CONDITIONS

flex provides a mechanism for conditionally
activating rules. Any rule whose pattern is prefixed with
''

will be active only when the scanner is in the

will be active only when the current start condition is either

Start conditions are declared in the definitions (first)
section of the input using unindented lines beginning with
either %s or %x followed by a list of names.
The former declares inclusive start conditions, the
latter exclusive start conditions. A start condition
is activated using the BEGIN action. Until the next
BEGIN action is executed, rules with the given start
condition will be active and rules with other start
conditions will be inactive. If the start condition is
inclusive, then rules with no start conditions at all
will also be active. If it is exclusive, then
only rules qualified with the start condition will be
active. A set of rules contingent on the same exclusive
start condition describe a scanner which is independent of
any of the other rules in the flex input. Because of
this, exclusive start conditions make it easy to specify
''

If the distinction between inclusive and exclusive start
conditions is still a little vague, here's a simple example
illustrating the connection between the two. The set of
rules:

%s example
%%
is equivalent to

%x example
%%
Without the qualifier, the bar pattern in the second example wouldn't be active (i.e., couldn't match) when in start condition example. If we just used to qualify bar, though, then it would only be active in example and not in INITIAL, while in the first example it's active in both, because in the first example the example startion condition is an inclusive(%s) start condition.

Also note that the special start-condition specifier
matches every start condition. Thus, the
above example could also have been written;

BEGIN(0) returns to the original state where only the rules with no start conditions are active. This state can also be referred to as the start-condition BEGIN(INITIAL) is equivalent to BEGIN(0). (The parentheses around the start condition name are not required but are considered good style.)

BEGIN actions can also be given as indented code at
the beginning of the rules section. For example, the
following will cause the scanner to enter the
yylex()
is called and the global variable enter_special is
true:

int enter_special;
%x SPECIAL
%%
if ( enter_special )
BEGIN(SPECIAL);
To illustrate the uses of start conditions, here is a scanner which provides two different interpretations of a string like

%{

include

Here is a scanner which recognizes (and discards) C comments while maintaining a count of the current input line.

%x comment
%%
int line_num = 1;
This scanner goes to a bit of trouble to match as much text as possible with each rule. In general, when attempting to write a high-speed scanner try to match as much possible in each rule, as it's a big win.

Note that start-conditions names are really integer values
and can be stored as such. Thus, the above could be extended
in the following fashion:

%x comment foo
%%
int line_num = 1;
int comment_caller;
Furthermore, you can access the current start condition using the integer-valued YY_START macro. For example, the above assignments to comment_caller could instead be written

comment_caller = YY_START;
Flex provides YYSTATE as an alias for YY_START (since that is what's used by AT__lex).''

Note that start conditions do not have their own name-space;
%s's and %x's declare names in the same fashion as

define's.

Finally, here's an example of how to match C-style quoted
strings using exclusive start conditions, including expanded
escape sequences (but not including checking for a string
that's too long):

%x str
%%
char string_buf[MAX_STR_CONST?;
char *string_buf_ptr;
Often, such as in some of the examples above, you wind up writing a whole bunch of rules all preceded by the same start condition(s). Flex makes this a little easier and cleaner by introducing a notion of start condition scope. A start condition scope is begun with:

where SCs is a list of one or more start conditions. Inside the start condition scope, every rule automatically has the prefix applied to it, until a '}' which matches the initial '{'. So, for example,

is equivalent to:

Start condition scopes may be nested.

Three routines are available for manipulating stacks of
start conditions:

void yy_push_state(int new_state)

pushes the current start condition onto the top of the start
condition stack and switches to new_state as though
you had used BEGIN new_state (recall that start
condition names are also integers).

void yy_pop_state()

pops the top of the stack and switches to it via
BEGIN.

int yy_top_state()

returns the top of the stack without altering the stack's
contents.

The start condition stack grows dynamically and so has no
built-in size limitation. If memory is exhausted, program
execution aborts.

To use start condition stacks, your scanner must include a
%option stack directive (see Options
below).

MULTIPLE INPUT BUFFERS

Some scanners (such as those which support
flex'' scanners do a large amount of
buffering, one cannot control where the next input will be
read from by simply writing a YY_INPUT which is
sensitive to the scanning context. YY_INPUT is only
called when the scanner reaches the end of its buffer, which
may be a long time after scanning a statement such as an
__

To negotiate these sorts of problems, flex provides a
mechanism for creating and switching between multiple input
buffers. An input buffer is created by using:

YY_BUFFER_STATE yy_create_buffer( FILE *file, int size )
which takes a FILE pointer and a size and creates a buffer associated with the given file and large enough to hold size characters (when in doubt, use YY_BUF_SIZE for the size). It returns a YY_BUFFER_STATE handle, which may then be passed to other routines (see below). The YY_BUFFER_STATE type is a pointer to an opaque struct yy_buffer_state structure, so you may safely initialize YY_BUFFER_STATE variables to ((YY_BUFFER_STATE) 0) if you wish, and also refer to the opaque structure in order to correctly declare input buffers in source files other than that of your scanner. Note that the FILE pointer in the call to yy_create_buffer is only used as the value of yyin seen by YY_INPUT; if you redefine YY_INPUT so it no longer uses yyin, then you can safely pass a nil FILE pointer to yy_create_buffer. You select a particular buffer to scan from using:

void yy_switch_to_buffer( YY_BUFFER_STATE new_buffer )
switches the scanner's input buffer so subsequent tokens will come from new_buffer. Note that yy_switch_to_buffer() may be used by yywrap() to set things up for continued scanning, instead of opening a new file and pointing yyin at it. Note also that switching input sources via either yy_switch_to_buffer() or yywrap() does not change the start condition.

void yy_delete_buffer( YY_BUFFER_STATE buffer )
is used to reclaim the storage associated with a buffer. ( buffer can be nil, in which case the routine does nothing.) You can also clear the current contents of a buffer using:

void yy_flush_buffer( YY_BUFFER_STATE buffer )
This function discards the buffer's contents, so the next time the scanner attempts to match a token from the buffer, it will first fill the buffer anew using YY_INPUT.

yy_new_buffer() is an alias for
yy_create_buffer(), provided for compatibility with
the C++ use of new and delete for creating and
destroying dynamic objects.

Finally, the YY_CURRENT_BUFFER macro returns a
YY_BUFFER_STATE handle to the current
buffer.

Here is an example of using these features for writing a
scanner which expands include files (the
feature is discussed
below):

/* the
Three routines are available for setting up input buffers for scanning in-memory strings instead of files. All of them create a new input buffer for scanning the string, and return a corresponding YY_BUFFER_STATE handle (which you should delete with yy_delete_buffer() when done with it). They also switch to the new buffer using yy_switch_to_buffer(), so the next call to yylex() will start scanning the string.

Note that both of these functions create and scan a
copy of the string or bytes. (This may be desirable,
since yylex() modifies the contents of the buffer it
is scanning.) You can avoid the copy by using:

yy_scan_buffer(char *base, yy_size_t
size)

which scans in place the buffer starting at base,
consisting of size bytes, the last two bytes of which
must be YY_END_OF_BUFFER_CHAR (ASCII NUL).
These last two bytes are not scanned; thus, scanning
consists of base[0? through base[size-2?,
inclusive.

If you fail to set up base in this manner (i.e.,
forget the final two YY_END_OF_BUFFER_CHAR bytes),
then yy_scan_buffer() returns a nil pointer instead
of creating a new input buffer.

The type yy_size_t is an integral type to which you
can cast an integer expression reflecting the size of the
buffer.

END-OF-FILE RULES

The special rule

-

assigning yyin to a new input file (in previous
versions of flex, after doing the assignment you had to call
the special action YY_NEW_FILE; this is no longer
necessary);

-

executing a return statement;

-

executing the special yyterminate()
action;

-

or, switching to a new buffer using
yy_switch_to_buffer() as shown in the example
above.

all start conditions which do
not already have

These rules are useful for catching things like unclosed comments. An example:

%x quote
%%
...other rules for dealing with quotes...

MISCELLANEOUS MACROS

The macro YY_USER_ACTION can be defined to provide an
action which is always executed prior to the matched rule's
action. For example, it could be #define'd to call a routine
to convert yytext to lower-case. When YY_USER_ACTION
is invoked, the variable yy_act gives the number of
the matched rule (rules are numbered starting with 1).
Suppose you want to profile how often each of your rules is
matched. The following would do the trick:

where ctr is an array to hold the counts for the different rules. Note that the macro YY_NUM_RULES gives the total number of rules (including the default rule, even if you use -s), so a correct declaration for ctr is:

int ctr[YY_NUM_RULES?;
The macro YY_USER_INIT may be defined to provide an action which is always executed before the first scan (and before the scanner's internal initializations are done). For example, it could be used to call a routine to read in a data table or open a logging file.

The macro yy_set_interactive(is_interactive) can be
used to control whether the current buffer is considered
interactive. An interactive buffer is processed more
slowly, but must be used when the scanner's input source is
indeed interactive to avoid problems due to waiting to fill
buffers (see the discussion of the -I flag below). A
non-zero value in the macro invocation marks the buffer as
interactive, a zero value as non-interactive. Note that use
of this macro overrides %option always-interactive or
%option never-interactive (see Options below).
yy_set_interactive() must be invoked prior to
beginning to scan the buffer that is (or is not) to be
considered interactive.

The macro yy_set_bol(at_bol) can be used to control
whether the current buffer's scanning context for the next
token match is done as though at the beginning of a line. A
non-zero macro argument makes rules anchored with '^'
active, while a zero argument makes '^' rules
inactive.

The macro YY_AT_BOL() returns true if the next token
scanned from the current buffer will have '^' rules active,
false otherwise.

In the generated scanner, the actions are all gathered in
one large switch statement and separated using
YY_BREAK, which may be redefined. By default, it is
simply a
YY_BREAK
allows, for example, C++ users to #define YY_BREAK to do
nothing (while being very careful that every rule ends with
a
YY_BREAK is inaccessible.

VALUES AVAILABLE TO THE USER

This section summarizes the various values available to the
user in the rule actions.

-

char *yytext holds the text of the current token. It
may be modified but not lengthened (you cannot append
characters to the end).

If the special directive %array appears in the first
section of the scanner description, then yytext is
instead declared char yytext[YYLMAX?, where
YYLMAX is a macro definition that you can redefine in
the first section if you don't like the default value
(generally 8KB). Using %array results in somewhat
slower scanners, but the value of yytext becomes
immune to calls to input() and unput(), which
potentially destroy its value when yytext is a
character pointer. The opposite of %array is
%pointer, which is the default.

You cannot use %array when generating C++ scanner
classes (the -+ flag).

-

int yyleng holds the length of the current
token.

-

FILE *yyin is the file which by default flex
reads from. It may be redefined but doing so only makes
sense before scanning begins or after an EOF has been
encountered. Changing it in the midst of scanning will have
unexpected results since flex buffers its input; use
yyrestart() instead. Once scanning terminates because
an end-of-file has been seen, you can assign yyin at
the new input file and then call the scanner again to
continue scanning.

-

void yyrestart( FILE *new_file ) may be called to
point yyin at the new input file. The switch-over to
the new file is immediate (any previously buffered-up input
is lost). Note that calling yyrestart() with
yyin as an argument thus throws away the current
input buffer and continues scanning the same input
file.

-

FILE *yyout is the file to which ECHO actions
are done. It can be reassigned by the user.

-

YY_CURRENT_BUFFER returns a YY_BUFFER_STATE
handle to the current buffer.

-

YY_START returns an integer value corresponding to
the current start condition. You can subsequently use this
value with BEGIN to return to that start
condition.

INTERFACING WITH YACC

One of the main uses of flex is as a companion to the
yacc parser-generator. yacc parsers expect to
call a routine named yylex() to find the next input
token. The routine is supposed to return the type of the
next token as well as putting any associated value in the
global yylval. To use flex with yacc,
one specifies the -d option to yacc to
instruct it to generate the file y.tab.h containing
definitions of all the %tokens appearing in the
yacc input. This file is then included in the
flex scanner. For example, if one of the tokens is
''

%{

include

OPTIONS

flex has the following options:

-b

Generate backing-up information to lex.backup. This
is a list of scanner states which require backing up and the
input characters on which they do so. By adding rules one
can remove backing-up states. If all backing-up
states are eliminated and -Cf or -CF is used,
the generated scanner will run faster (see the -p
flag). Only users who wish to squeeze every last cycle out
of their scanners need worry about this option. (See the
section on Performance Considerations below.)

-c

is a do-nothing, deprecated option included for POSIX
compliance.

-d

makes the generated scanner run in debug mode.
Whenever a pattern is recognized and the global
yy_flex_debug is non-zero (which is the default), the
scanner will write to stderr a line of the
form:

--accepting rule at line 53 (
The line number refers to the location of the rule in the file defining the scanner (i.e., the file that was fed to flex). Messages are also generated when the scanner backs up, accepts the default rule, reaches the end of its input buffer (or encounters a NUL; at this point, the two look the same as far as the scanner's concerned), or reaches an end-of-file.

-f

specifies fast scanner. No table compression is done
and stdio is bypassed. The result is large but fast. This
option is equivalent to -Cfr (see
below).

-h

generates a flex's
options to stdout'' and then exits. -? and
--help are synonyms for -h.

-i

instructs flex to generate a case-insensitive
scanner. The case of letters given in the flex input
patterns will be ignored, and tokens in the input will be
matched regardless of case. The matched text given in
yytext will have the preserved case (i.e., it will
not be folded).

-l

turns on maximum compatibility with the original AT
lex implementation. Note that this does not mean
full'' compatibility. Use of this option costs a
considerable amount of performance, and it cannot be used
with the -+, -f, -F, -Cf, or -CF options. For
details on the compatibilities it provides, see the section
YY_FLEX_LEX_COMPAT
being #define'd in the generated scanner.

-n

is another do-nothing, deprecated option included only for
POSIX compliance.

-p

generates a performance report to stderr. The report
consists of comments regarding features of the flex
input file which will cause a serious loss of performance in
the resulting scanner. If you give the flag twice, you will
also get comments regarding features that lead to minor
performance losses.

Note that the use of REJECT, %option yylineno, and
variable trailing context (see the Deficiencies / Bugs
section below) entails a substantial performance penalty;
use of yymore(), the ^ operator, and the
-I flag entail minor performance
penalties.

-s

causes the default rule (that unmatched scanner input
is echoed to stdout) to be suppressed. If the scanner
encounters input that does not match any of its rules, it
aborts with an error. This option is useful for finding
holes in a scanner's rule set.

-t

instructs flex to write the scanner it generates to
standard output instead of lex.yy.c.

-v

specifies that flex should write to stderr a
summary of statistics regarding the scanner it generates.
Most of the statistics are meaningless to the casual
flex user, but the first line identifies the version
of flex (same as reported by -V), and the next
line the flags used when generating the scanner, including
those that are on by default.

-w

suppresses warning messages.

-B

instructs flex to generate a batch scanner,
the opposite of interactive scanners generated by
-I (see below). In general, you use -B when
you are certain that your scanner will never be used
interactively, and you want to squeeze a little more
performance out of it. If your goal is instead to squeeze
out a lot more performance, you should be using the
-Cf or -CF options (discussed below), which
turn on -B automatically anyway.

-F

specifies that the fast scanner table representation
should be used (and stdio bypassed). This representation is
about as fast as the full table representation (-f),
and for some sets of patterns will be considerably smaller
(and for others, larger). In general, if the pattern set
contains both
__

then you're better off using the full table representation. If only the -F.__

This option is equivalent to -CFr (see below). It
cannot be used with -+.

-I

instructs flex to generate an interactive
scanner. An interactive scanner is one that only looks ahead
to decide what token has been matched if it absolutely must.
It turns out that always looking one extra character ahead,
even if the scanner has already seen enough text to
disambiguate the current token, is a bit faster than only
looking ahead when necessary. But scanners that always look
ahead give dreadful interactive performance; for example,
when a user types a newline, it is not recognized as a
newline token until they enter another token, which
often means typing in another whole line.

Flex scanners default to interactive unless
you use the -Cf or -CF table-compression
options (see below). That's because if you're looking for
high-performance you should be using one of these options,
so if you didn't, flex assumes you'd rather trade off
a bit of run-time performance for intuitive interactive
behavior. Note also that you cannot use -I in
conjunction with -Cf or -CF. Thus, this option
is not really needed; it is on by default for all those
cases in which it is allowed.

You can force a scanner to not be interactive by
using -B (see above).

-L

instructs flex not to generate #line
directives. Without this option, flex peppers the
generated scanner with #line directives so error messages in
the actions will be correctly located with respect to either
the original flex input file (if the errors are due
to code in the input file), or lex.yy.c (if the
errors are flex's fault -- you should report these
sorts of errors to the email address given
below).

-T

makes flex run in trace mode. It will generate
a lot of messages to stderr concerning the form of
the input and the resultant non-deterministic and
deterministic finite automata. This option is mostly for use
in maintaining flex.

-V

prints the version number to stdout and exits.
--version is a synonym for -V.

-7

instructs flex to generate a 7-bit scanner, i.e., one
which can only recognized 7-bit characters in its input. The
advantage of using -7 is that the scanner's tables
can be up to half the size of those generated using the
-8 option (see below). The disadvantage is that such
scanners often hang or crash if their input contains an
8-bit character.

Note, however, that unless you generate your scanner using
the -Cf or -CF table compression options, use
of -7 will save only a small amount of table space,
and make your scanner considerably less portable.
Flex's default behavior is to generate an 8-bit
scanner unless you use the -Cf or -CF, in
which case flex defaults to generating 7-bit scanners
unless your site was always configured to generate 8-bit
scanners (as will often be the case with non-USA sites). You
can tell whether flex generated a 7-bit or an 8-bit scanner
by inspecting the flag summary in the -v output as
described above.

Note that if you use -Cfe or -CFe (those table
compression options, but also using equivalence classes as
discussed see below), flex still defaults to generating an
8-bit scanner, since usually with these compression options
full 8-bit tables are not much more expensive than 7-bit
tables.

-8

instructs flex to generate an 8-bit scanner, i.e.,
one which can recognize 8-bit characters. This flag is only
needed for scanners generated using -Cf or
-CF, as otherwise flex defaults to generating an
8-bit scanner anyway.

See the discussion of -7 above for flex's default
behavior and the tradeoffs between 7-bit and 8-bit
scanners.

-+

specifies that you want flex to generate a C++ scanner
class. See the section on Generating C++ Scanners below for
details.

controls the degree of table compression and, more
generally, trade-offs between small scanners and fast
scanners.

-Ca (
__

-Ce directs flex to construct equivalence
classes, i.e., sets of characters which have identical
lexical properties (for example, if the only appearance of
digits in the flex input is in the character class
''

-Cf specifies that the full scanner tables
should be generated - flex should not compress the
tables by taking advantages of similar transition functions
for different states.

-CF specifies that the alternate fast scanner
representation (described above under the -F flag)
should be used. This option cannot be used with
-+.

-Cm directs flex to construct
meta-equivalence classes, which are sets of
equivalence classes (or characters, if equivalence classes
are not being used) that are commonly used together.
Meta-equivalence classes are often a big win when using
compressed tables, but they have a moderate performance
impact (one or two
''

-Cr causes the generated scanner to bypass use
of the standard I/O library (stdio) for input. Instead of
calling fread() or getc(), the scanner will
use the read() system call, resulting in a
performance gain which varies from system to system, but in
general is probably negligible unless you are also using
-Cf or -CF. Using -Cr can cause strange
behavior if, for example, you read from yyin using
stdio prior to calling the scanner (because the scanner will
miss whatever text your previous reads left in the stdio
input buffer).

-Cr has no effect if you define YY_INPUT (see
The Generated Scanner above).

A lone -C specifies that the scanner tables should be
compressed but neither equivalence classes nor
meta-equivalence classes should be used.

The options -Cf or -CF and -Cm do not
make sense together - there is no opportunity for
meta-equivalence classes if the table is not being
compressed. Otherwise the options may be freely mixed, and
are cumulative.

The default setting is -Cem, which specifies that
flex should generate equivalence classes and
meta-equivalence classes. This setting provides the highest
degree of table compression. You can trade off
faster-executing scanners at the cost of larger tables with
the following generally being true:

slowest
Note that scanners with the smallest tables are usually generated and compiled the quickest, so during development you will usually want to use the default, maximal compression.

-Cfe is often a good compromise between speed and
size for production scanners.

-ooutput

directs flex to write the scanner to the file output
instead of lex.yy.c. If you combine -o with
the -t option, then the scanner is written to
stdout but its #line directives (see the
-L option above) refer to the file
output.

-Pprefix

changes the default yy prefix used by flex for
all globally-visible variable and function names to instead
be prefix. For example, -Pfoo changes the name
of yytext to footext. It also changes the name
of the default output file from lex.yy.c to
lex.foo.c. Here are all of the names
affected:

yy_create_buffer
yy_delete_buffer
yy_flex_debug
yy_init_buffer
yy_flush_buffer
yy_load_buffer_state
yy_switch_to_buffer
yyin
yyleng
yylex
yylineno
yyout
yyrestart
yytext
yywrap
(If you are using a C++ scanner, then only yywrap and yyFlexLexer are affected.) Within your scanner itself, you can still refer to the global variables and functions using either version of their name; but externally, they have the modified name.

This option lets you easily link together multiple
flex programs into the same executable. Note, though,
that using this option also renames yywrap(), so you
now must either provide your own
(appropriately-named) version of the routine for your
scanner, or use %option noyywrap, as linking with
-lfl no longer provides one for you by
default.

-Sskeleton_file

overrides the default skeleton file from which flex
constructs its scanners. You'll never need this option
unless you are doing flex maintenance or
development.

flex also provides a mechanism for controlling
options within the scanner specification itself, rather than
from the flex command-line. This is done by including
%option directives in the first section of the
scanner specification. You can specify multiple options with
a single %option directive, and multiple directives
in the first section of your flex input file.

Most options are given simply as names, optionally preceded
by the word

instructs flex to generate a scanner which always considers
its input
isatty()__ in an attempt
to determine whether the scanner's input source is
interactive and thus should be read a character at a time.
When this option is used, however, then no such call is
made.

main

directs flex to provide a default main() program for
the scanner, which simply calls yylex(). This option
implies noyywrap (see below).

never-interactive

instructs flex to generate a scanner which never considers
its input
isatty()). This is the opposite of
always-interactive.__

if set (i.e., %option stdinit) initializes
yyin and yyout to stdin and
stdout, instead of the default of nil. Some
existing lex programs depend on this behavior, even
though it is not compliant with ANSI C, which does not
require stdin and stdout to be compile-time
constant. In a reentrant scanner, however, this is not a
problem since initialization is performed in
yylex_init at runtime.

yylineno

directs flex to generate a scanner that maintains the
number of the current line read from its input in the global
variable yylineno. This option is implied by
%option lex-compat.

yywrap

if unset (i.e., %option noyywrap), makes the scanner
not call yywrap() upon an end-of-file, but simply
assume that there are no more files to scan (until the user
points yyin at a new file and calls yylex()
again).

flex scans your rule actions to determine whether you
use the REJECT or yymore() features. The
reject and yymore options are available to
override its decision as to whether you use the options,
either by setting them (e.g., %option reject) to
indicate the feature is indeed used, or unsetting them to
indicate it actually is not used (e.g., %option
noyymore).

Three options take string-delimited values, offset with
'=':

%option outfile=
is equivalent to -oABC, and

%option prefix=
is equivalent to -PXYZ. Finally,

%option yyclass=
only applies when generating a C++ scanner ( -+ option). It informs flex that you have derived foo as a subclass of yyFlexLexer, so flex will place your actions in the member function foo::yylex() instead of yyFlexLexer::yylex(). It also generates a yyFlexLexer::yylex() member function that emits a run-time error (by invoking yyFlexLexer::!LexerError?()) if called. See Generating C++ Scanners, below, for additional information.

A number of options are available for lint purists who want
to suppress the appearance of unneeded routines in the
generated scanner. Each of the following, if unset (e.g.,
%option nounput ), results in the corresponding
routine not appearing in the generated scanner:

PERFORMANCE CONSIDERATIONS

The main design goal of flex is that it generate
high-performance scanners. It has been optimized for dealing
well with large sets of rules. Aside from the effects on
scanner speed of the table compression -C options
outlined above, there are a number of options/actions which
degrade performance. These are, from most expensive to
least:

REJECT
%option yylineno
arbitrary trailing context
pattern sets that require backing up
%array
%option interactive
%option always-interactive
'^' beginning-of-line operator
yymore()
with the first three all being quite expensive and the last two being quite cheap. Note also that unput() is implemented as a routine call that potentially does quite a bit of work, while yyless() is a quite-cheap macro; so if just putting back some excess text you scanned, use yyless().

REJECT should be avoided at all costs when
performance is important. It is a particularly expensive
option.

Getting rid of backing up is messy and often may be an
enormous amount of work for a complicated scanner. In
principal, one begins by using the -b flag to
generate a lex.backup file. For example, on the
input

State #6 is non-accepting -
associated rule line numbers:
2 3
out-transitions: [ o?
jam-transitions: EOF [ 001-n p-177?
State #8 is non-accepting -
associated rule line numbers:
3
out-transitions: [ a?
jam-transitions: EOF [ 001-` b-177?
State #9 is non-accepting -
associated rule line numbers:
3
out-transitions: [ r?
jam-transitions: EOF [ 001-q s-177?
Compressed tables always back up.
The first few lines tell us that there's a scanner state in which it can make a transition on an 'o' but not on any other character, and that in that state the currently scanned text does not match any rule. The state occurs when trying to match the rules found at lines 2 and 3 in the input file. If the scanner is in that state and then reads something other than an 'o', it will have to back up to find a rule which is matched. With a bit of headscratching one can see that this must be the state it's in when it has seen

The comment regarding State #8 indicates there's a problem
when

The final comment reminds us that there's no point going to
all the trouble of removing backing up from the rules unless
we're using -Cf or -CF, since there's no
performance gain doing so with compressed
scanners.

%%
foo return TOK_KEYWORD;
foobar return TOK_KEYWORD;
[a-z?+ return TOK_ID;
This is usually the best solution when appropriate.

Backing up messages tend to cascade. With a complicated set
of rules it's not uncommon to get hundreds of messages. If
one can decipher them, though, it often only takes a dozen
or so rules to eliminate the backing up (though it's easy to
make a mistake and have an error rule accidentally match a
valid token. A possible future flex feature will be
to automatically add rules to eliminate backing
up).

It's important to keep in mind that you gain the benefits of
eliminating backing up only if you eliminate every
instance of backing up. Leaving just one means you gain
nothing.

Variable trailing context (where both the leading and
trailing parts do not have a fixed length) entails almost
the same performance loss as REJECT (i.e.,
substantial). So when possible a rule like:

%%
mouse|rat/(cat|dog) run();
is better written:

%%
mouse/cat|dog run();
rat/cat|dog run();
or as

%%
mouse|rat/cat run();
mouse|rat/dog run();
Note that here the special '|' action does not provide any savings, and can even make things worse (see Deficiencies / Bugs below).

Another area where the user can increase a scanner's
performance (and one that's easier to implement) arises from
the fact that the longer the tokens matched, the faster the
scanner will run. This is because with long tokens the
processing of most input characters takes place in the
(short) inner scanning loop, and does not often have to go
through the additional work of setting up the scanning
environment (e.g., yytext) for the action. Recall the
scanner for C comments:

%x comment
%%
int line_num = 1;
This could be sped up by writing it as:

%x comment
%%
int line_num = 1;
Now instead of each newline requiring the processing of another action, recognizing the newlines is adding rules does not'' slow down the scanner! The speed of the scanner is independent of the number of rules or (modulo the considerations given at the beginning of this section) how complicated the rules are with regard to operators such as '*' and '|'.

A final example in speeding up a scanner: suppose you want
to scan through a file containing identifiers and keywords,
one per line and with no other extraneous characters, and
recognize all the keywords. A natural first approach
is:

%%
asm |
auto |
break |
... etc ...
volatile |
while /* it's a keyword /
[a-z?+ |
.|n / it's not a keyword */
Now, if it's guaranteed that there's exactly one word per line, then we can reduce the total number of matches by a half by merging in the recognition of newlines with that of the other tokens:

%%
asmn |
auton |
breakn |
... etc ...
volatilen |
whilen /* it's a keyword /
[a-z?+n |
.|n / it's not a keyword */
One has to be careful here, as we have now reintroduced backing up into the scanner. In particular, while we know that there will never be any characters in the input stream other than letters or newlines, flex can't figure this out, and it will plan for possibly needing to back up when it has scanned a token like ''

%%
asmn |
auton |
breakn |
... etc ...
volatilen |
whilen /* it's a keyword /
[a-z?+n |
[a-z?+ |
.|n / it's not a keyword */
Compiled with -Cf, this is about as fast as one can get a flex scanner to go for this particular problem.

A final note: flex is slow when matching NUL's,
particularly when a token contains multiple NUL's. It's best
to write rules which match short amounts of text if
it's anticipated that the text will often include
NUL's.

Another final note regarding performance: as mentioned above
in the section How the Input is Matched, dynamically
resizing yytext to accommodate huge tokens is a slow
process because it presently requires that the (huge) token
be rescanned from the beginning. Thus if performance is
vital, you should attempt to match
__

GENERATING C++ SCANNERS

flex provides two different ways to generate scanners
for use with C++. The first way is to simply compile a
scanner generated by flex using a C++ compiler
instead of a C compiler. You should not encounter any
compilations errors (please report any you find to the email
address given in the Author section below). You can then use
C++ code in your rule actions instead of C code. Note that
the default input source for your scanner remains
yyin, and default echoing is still done to
yyout. Both of these remain FILE * variables
and not C++ streams.

You can also use flex to generate a C++ scanner
class, using the -+ option (or, equivalently,
%option c++), which is automatically specified if the
name of the flex executable ends in a '+', such as
flex++. When using this option, flex defaults to
generating the scanner to the file lex.yy.cc instead
of lex.yy.c. The generated scanner includes the
header file !FlexLexer?.h, which defines the interface
to two C++ classes.

The first class, !FlexLexer?, provides an abstract base
class defining the general scanner class interface. It
provides the following member functions:

const char* YYText()

returns the text of the most recently matched token, the
equivalent of yytext.

int YYLeng()

returns the length of the most recently matched token, the
equivalent of yyleng.

int lineno() const

returns the current input line number (see %option
yylineno), or 1 if %option yylineno was
not used.

void set_debug( int flag )

sets the debugging flag for the scanner, equivalent to
assigning to yy_flex_debug (see the Options section
above). Note that you must build the scanner using
%option debug to include debugging information in
it.

int debug() const

returns the current setting of the debugging
flag.

Also provided are member functions equivalent to
yy_switch_to_buffer(), yy_create_buffer() (though the
first argument is an istream* object pointer and not
a FILE*), yy_flush_buffer(), yy_delete_buffer(), and
yyrestart() (again, the first argument is a
istream* object pointer).

The second class defined in !FlexLexer?.h is
yyFlexLexer, which is derived from !FlexLexer?.
It defines the following additional member
functions:

yyFlexLexer( istream* arg_yyin = 0, ostream* arg_yyout =
0 )

constructs a yyFlexLexer object using the given
streams for input and output. If not specified, the streams
default to cin and cout,
respectively.

virtual int yylex()

performs the same role is yylex() does for ordinary
flex scanners: it scans the input stream, consuming tokens,
until a rule's action returns a value. If you derive a
subclass S from yyFlexLexer and want to access
the member functions and variables of S inside
yylex(), then you need to use %option
yyclass= to inform flex that you
will be using that subclass instead of yyFlexLexer.
In this case, rather than generating
yyFlexLexer::yylex(),flex generates
S::yylex() (and also generates a dummy
yyFlexLexer::yylex() that calls
yyFlexLexer::!LexerError?() if called).

reads up to max_size characters into buf and
returns the number of characters read. To indicate
end-of-input, return 0 characters. Note that
-B and
-I flags) define the macro YY_INTERACTIVE. If
you redefine !LexerInput?() and need to take different
actions depending on whether or not the scanner might be
scanning an interactive input source, you can test for the
presence of this name via #ifdef.

reports a fatal error message. The default version of this
function writes the message to the stream cerr and
exits.

Note that a yyFlexLexer object contains its
entire scanning state. Thus you can use such objects
to create reentrant scanners. You can instantiate multiple
instances of the same yyFlexLexer class, and you can
also combine multiple C++ scanner classes together in the
same program using the -P option discussed
above.

Finally, note that the %array feature is not
available to C++ scanner classes; you must use
%pointer (the default).

Here is an example of a simple C++ scanner:

// An example of using the flex C++ scanner class.
%{
int mylineno = 0;
%}
string
If you want to create multiple (different) lexer classes, you use the -P flag (or the prefix= option) to rename each yyFlexLexer to some other xxFlexLexer. You then can include in your other sources once per lexer class, first renaming yyFlexLexer as follows:

undef yyFlexLexer

define yyFlexLexer xxFlexLexer

include

if, for example, you used %option prefix= for one of your scanners and %option prefix= for the other.

IMPORTANT: the present form of the scanning class is
experimental and may change considerably between
major releases.

INCOMPATIBILITIES WITH LEX AND POSIX

flex is a rewrite of the ATlex
tool (the two implementations do not share any code,
though), with some extensions and incompatibilities, both of
which are of concern to those who wish to write scanners
acceptable to either implementation. Flex is fully compliant
with the POSIX lex specification, except that when
using %pointer (the default), a call to
unput() destroys the contents of yytext, which
is counter to the POSIX specification.

In this section we discuss all of the known areas of
incompatibility between flex, AT

flex's-l option turns on maximum
compatibility with the original ATlex''
implementation, at the cost of a major loss in the generated
scanner's performance. We note below which incompatibilities
can be overcome using the -l__ option.

yylineno should be maintained on a per-buffer basis,
rather than a per-scanner (single global variable)
basis.

yylineno is not part of the POSIX
specification.

-

The input() routine is not redefinable, though it may
be called to read characters following whatever has been
matched by a rule. If input() encounters an
end-of-file the normal yywrap() processing is done. A
``real end-of-file is returned by input() as
EOF.''

Input is instead controlled by defining the YY_INPUT
macro.

The flex restriction that input() cannot be
redefined is in accordance with the POSIX specification,
which simply does not specify any way of controlling the
scanner's input other than by making an initial assignment
to yyin.

-

The unput() routine is not redefinable. This
restriction is in accordance with POSIX.

-

flex scanners are not as reentrant as lex
scanners. In particular, if you have an interactive scanner
and an interrupt handler which long-jumps out of the
scanner, and the scanner is subsequently called again, you
may get the following message:

fatal flex scanner internal error--end of buffer missed
To reenter the scanner, first use

yyrestart( yyin );
Note that this call will throw away any buffered input; usually this isn't a problem with an interactive scanner.

Also note that flex C++ scanner classes are
reentrant, so if using C++ is an option for you, you should
use them instead. See
''

-

output() is not supported. Output from the
ECHO macro is done to the file-pointer yyout
(default stdout).

output() is not part of the POSIX
specification.

-

lex does not support exclusive start conditions (%x),
though they are in the POSIX specification.

-

When definitions are expanded, flex encloses them in
parentheses. With lex, the following:

NAME [A-Z?[A-Z0-9?*
%%
foo{NAME}? printf(
will not match the string flex, the rule will be expanded to

Note that if the definition begins with ^ or ends
with $ then it is not expanded with
parentheses, to allow these operators to appear in
definitions without losing their special meanings. But the
and
operators cannot be used in a flex
definition.

Using -l results in the lex behavior of no
parentheses around the definition.

The POSIX specification is that the definition be enclosed
in parentheses.

-

Some implementations of lex allow a rule's action to
begin on a separate line, if the rule's pattern has trailing
whitespace:

%%
foo|bar
flex does not support this feature.

-

The lex%r (generate a Ratfor scanner) option
is not supported. It is not part of the POSIX
specification.

-

After a call to unput(),yytext is undefined
until the next token is matched, unless the scanner was
built using %array. This is not the case with
lex or the POSIX specification. The -l option
does away with this incompatibility.

-

The precedence of the {} (numeric range) operator is
different. lex interprets
flex interprets it as
''

-

The precedence of the ^ operator is different.
lex interprets
flex interprets it as
''

-

The special table-size declarations such as %a
supported by lex are not required by flex
scanners; flex ignores them.

-

The name FLEX_SCANNER is #define'd so scanners may be
written for use with either flex or lex.
Scanners also include YY_FLEX_MAJOR_VERSION and
YY_FLEX_MINOR_VERSION indicating which version of
flex generated the scanner (for example, for the 2.5
release, these defines would be 2 and 5
respectively).

The following flex features are not included in
lex or the POSIX specification:

C++ scanners
%option
start condition scopes
start condition stacks
interactive/non-interactive scanners
yy_scan_string() and friends
yyterminate()
yy_set_interactive()
yy_set_bol()
YY_AT_BOL()
plus almost all of the flex flags. The last feature in the list refers to the fact that with flex you can put multiple actions on the same line, separated with semi-colons, while with lex, the following

foo handle_foo();
flex does not truncate the action. Actions that are not enclosed in braces are simply terminated at the end of the line.

DIAGNOSTICS

warning, rule cannot be matched indicates that the
given rule cannot be matched because it follows other rules
that will always match the same text as it. For example, in
the following
''

[a-z?+ got_identifier();
foo got_foo();
Using REJECT in a scanner suppresses this warning.

warning,-soption given but default rule
can be matched means that it is possible (perhaps only
in a particular start condition) that the default rule
(match any single character) is the only one that will match
a particular input. Since -s was given, presumably
this is not intended.

reject_used_but_not_detected undefined or
yymore_used_but_not_detected undefined - These errors
can occur at compile time. They indicate that the scanner
uses REJECT or yymore() but that flex
failed to notice the fact, meaning that flex scanned
the first two sections looking for occurrences of these
actions and failed to find any, but somehow you snuck some
in (via a #include file, for example). Use %option
reject or %option yymore to indicate to flex that
you really do use these features.

flex scanner jammed - a scanner compiled with
-s has encountered an input string which wasn't
matched by any of its rules. This error can also occur due
to internal problems.

token too large, exceeds YYLMAX - your scanner uses
%array and one of its rules matched a string longer
than the YYLMAX constant (8K bytes by default). You
can increase the value by #define'ing YYLMAX in the
definitions section of your flex input.

scanner requires -8 flag to use the character 'x' -
Your scanner specification includes recognizing the 8-bit
character 'x' and you did not specify the -8 flag,
and your scanner defaulted to 7-bit because you used the
-Cf or -CF table compression options. See the
discussion of the -7 flag for details.

flex scanner push-back overflow - you used
unput() to push back so much text that the scanner's
buffer could not hold both the pushed-back text and the
current token in yytext. Ideally the scanner should
dynamically resize the buffer in this case, but at present
it does not.

input buffer overflow, can't enlarge buffer because
scanner uses REJECT - the scanner was working on
matching an extremely large token and needed to expand the
input buffer. This doesn't work with scanners that use
REJECT.

''fatal flex scanner internal error--end of buffer missed

'' This can occur in an scanner which is reentered after

a long-jump has jumped out (or over) the scanner's
activation frame. Before reentering the scanner,
use:

DEFICIENCIES / BUGS

For some trailing context rules, parts which are actually
fixed-length are not recognized as such, leading to the
abovementioned performance loss. In particular, parts using
'|' or {n} (such as

Combining trailing context with the special '|' action can
result in fixed trailing context being turned into
the more expensive variable trailing context. For
example, in the following:

%%
abc |
xyz/def
Use of unput() invalidates yytext and yyleng, unless the %array directive or the -l option has been used.

Pattern-matching of NUL's is substantially slower than
matching other characters.

Dynamic resizing of the input buffer is slow, as it entails
rescanning all the text matched so far by the current
(generally huge) token.

Due to both buffering of input and read-ahead, you cannot
intermix calls to
getchar(), with flex rules and expect
it to work. Call input()__ instead.

The total table entries listed by the -v flag
excludes the number of table entries needed to determine
what rule has been matched. The number of entries is equal
to the number of DFA states if the scanner does not use
REJECT, and somewhat greater than the number of
states if it does.

AUTHOR

Vern Paxson, with the help of many ideas and much
inspiration from Van Jacobson. Original version by Jef
Poskanzer. The fast table representation is a partial
implementation of a design done by Van Jacobson. The
implementation was done by Kevin Gong and Vern
Paxson.

Thanks to Esmond Pitt and Earle Horton for 8-bit character
support; to Benson Margulies and Fred Burke for C++ support;
to Kent Williams and Tom Epperly for C++ class support; to
Ove Ewerlid for support of NUL's; and to Eric Hughes for
support of multiple buffers.

This work was primarily done when I was with the Real Time
Systems Group at the Lawrence Berkeley Laboratory in
Berkeley, CA. Many thanks to all there for the support I
received.

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